The following technologies are known as those aimed at reducing fatigue by changing the sitting posture of a person sitting for a long time. First, a technology to change a sifting posture through changing a fixed position of a lumbar support provided on a vehicle seat, such as Patent Document 1 (Japanese Patent Application Laid-open No. Sho 64-44355).
Second, a technology to change the angle of the back (vicinity of the lumbar vertebra) and the femur (vicinity of the pelvis) of a seated person at the same angle while fixing the angle (positional relation) between the back and the femur through changing the supporting face angle of the seat cushion and the seat back, such as Patent Document 2 (Japanese Patent Application Laid-open No. Hei 4-224709).
Third, a technology to change a sitting posture through changing a seat shape using a means to perform fuzzy deduction from a driving speed and driving period of time such as Patent Document 3 (Japanese Patent Application Laid-open No. Hei 2-136340).
Fourth, a technology to change a sitting posture through changing the seat state using a means for detecting body motion and a means for supplementing body motion to detect a minute movement of a seated person such as Patent Document 4 (Japanese Patent Application Laid-open No. Hei 9-84656).
However, all of the technologies disclosed in the Patent Documents 1 to 3 are technologies applied to an ordinary urethane seat structured by placing an urethane material with a prescribed thickness on a seat spring (seat back spring, seat cushion spring). Accordingly, changing a sitting posture is to force a lumbar support, a side support, a front lifter, a rear lifter, a reclining device, etc., to move. When only the lumbar support or the side support is forced to move, a stimulus is leaned to a local area. This kind of unbalanced stimulus is effective at restoring awakenness rather than reducing fatigue, and when the local stimulus like this is maintained during a long period of driving, the feeling of fatigue is rather enhanced. Accordingly, Patent Document 1 copes with this disadvantage by forcing the movement of a lumbar support only when a prescribed length of elapsed time is detected.
In a structure to change a supporting face angle using a front lifter or a rear lifter as in the Patent Document 2, although no local stimulus like the above is not sensed, since angle change of a seat cushion or seat back is performed with an ordinary urethane seat, it is necessary to force not only a cushioning member (urethane) but also a cushion frame and a seat frame to move. The change of the supporting face angle is characterized by changing the supporting face angle while restraining deviation of the angle between the pelvis and the lumbar vertebra. Therefore, when the supporting face angle is changed, the angles of the back (vicinity of the lumbar vertebra) and the femur (vicinity of the pelvis) of the seated person to the horizontal surface also change, and no change occurs in the sitting posture itself. In other words, in the case of sitting with an extended back or in the case of sitting with round shoulders, the posture is kept as it is, and only the angles of the back and the femur to the horizontal surface change at the same angle. Therefore, in the technology described in the Patent Document 2, the sitting posture is substantially fixed, and a significant fatigue reduction effect cannot be expected.
On the other hand, in Patent Document 4, there is a description of a problem that with a mechanism to operate depending on a sitting period or to operate periodically, a seated person is forced to change a posture even when the seated person does not want to do it, which makes the seated person feel unpleasant. The Patent Document 4 is characterized by that when a seated person shows a body movement, the mechanism operates in a direction to promote the body movement. Body movement of the seated person's own accord is often seen when the feeling of fatigue or the like is elevated, and is excellent in view of effectively performing fatigue reduction. However, fatigue is naturally accumulated during no spontaneous body movement of a seated person. Therefore, it is preferable to have a structure possible to induce a posture change when a feeling of fatigue is raised, and at the same time possible to reduce accumulation of fatigue even when such a conscious fatigue feeling is not sensed, and possible to perform a minute posture change in a level that the seated person does not feel unpleasant.
The present invention is achieved from a view of the above-described problems, and an object thereof is to provide a seat structure which is able to change a supporting pressure not locally but over a wider area of a seat cushion or a seat back to stimulate blood stream and reduce fatigue, and is possible to minutely and surely perform sitting posture change without necessity of moving a cushion frame or a seat frame. In addition, it is also an object of the present invention to provide a seat structure which can temporarily restore awakeness when a degree of fatigue is raised or awakeness is lowered.